The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 6,853,093; 7,497,378; 7,784,691; 6,646,565;
U.S. Published Patent Application Nos. 2008/0180245; 2009/0058628 and 2009/0184850; and
GB2411756; EP1421549 and EP1676182.